This invention relates to an amplifier circuit for supplying a load with an output signal current proportional to an input signal voltage, and which is especially suitable for a recording amplifier of a tape recorder.
In FIG. 1 showing a basic circuit arrangement of a prior art amplifier circuit for supplying a load with an output signal current proportional to an input signal voltage, an input terminal 1 connected with a signal source (not shown) is connected to the inverting input of an operational or differential amplifier 2 through a resistor 3 having a resistance value of Ra. The noninverting input of operational amplifier 2 is connected to circuit ground. The output of amplifier 2 is connected to the inverting input of amplifier 2 via a load 4 to provide negative feedback.
In the aforesaid circuit, if an input signal voltage applied to the input terminal 1 is v.sub.a, a current i.sub.a flowing through the load 4 is given by ##EQU1## Namely, the current i.sub.a flowing through the load 4 is in proportion to the input signal voltage v.sub.a without regard to the impedance of the load 4. In other words, if the input signal voltage v.sub.a is constant, then the load current will be constant independently of the load impedance.
Although functioning as an amplifier, as described above, the circuit of FIG. 1 has such a disadvantage that one terminal of the load 4 cannot be connected to circuit ground because the load 4 is disposed in the feedback circuit of the amplifier 2.
FIG. 2 shows another prior art constant-current amplifier called a constant-current sink type constant-current amplifier, in which an input terminal 5 is connected to the noninverting input of an operational amplifier 6, whose output is connected to the base of a transistor 7. The emitter of transistor 7 is connected to the inverting input of amplifier 6 and also connected to a negative power supply terminal 9 through a resistor 8 with a resistance value of Rb, while the collector of transistor 7 is connected to a positive power supply terminal 10 through a load 11.
In such a circuit, the negative power terminal 9 may be regarded as circuit ground with respect to AC signals. Accordingly, if a base current of transistor 7 is ignored, a current i.sub.b flowing through the load 11 when a signal voltage v.sub.b is applied to the input terminal 5 will be given by ##EQU2##
Like the circuit of FIG. 1, the circuit of FIG. 2 functions as an amplifier. With such a drawback that the load current has a DC component because the direction of the instantaneous current flowing through the load 11 is always fixed, however, this circuit is not fit for use as a recording amplifier to supply a signal current to a recording head.